I would like to have a slideshow function. That is, set the cycle frequency temporarely
at a predefined numer of seconds. (yes I know what it does to the system performance..)
Preferably using a shortcut.

With the latest version you can set the cycling time down to 1 second. Of course, like you said, it might cause performance issues, especially with complicated layouts.
You can set the desired cycling time and then just use the already available shortcuts to start or stop cycling.

I understand that, but I would like to have a normal refresh rate of 10 minutes
and then when I start the 'slideshow option' set this refresh rate temporarely to
5 seconds (for example) by using a keyboard shortcut. When I want to end the
slideshow I press the same keyboard shortcut to reset the original 10 minutes refresh rate.

BTW I have a dual monitor setup and frquently use the second monitor to view my
photo's by manually advancing.